I am trying to standardize a stock solution of formaldehyde and I am about to go insane. I am following the method to the tee and it is simply not working out. This method is very famous and is the following:
1. Na2SO3 - 1 mol/L (eg. can be made by dissolving 126 g of anhydrous sodium sulfite per litre of water)
2. Thymolphthalein - 10 g/L of ethanol
3. H2SO4 - 0.01 mol/L (or 0.02 N)
4. 1500 ug/mL stock solution - 3.8 mL 37% v/v of formaldehyde solution made up to 1 L with water
Pipette 50 mL of sodium sulfite (1.) into 150 mL Erlenmeyer flask. Add two drops of thymolphthalein indicator (2.). Add a few drops of sulfuric acid (3.) if necessary until the blue colour disappears.
I follow the above. The problem starts when I place 50 mL of sodium sulfite into the E. flask and add the thymolphthalein. I add a couple of drops and the solution turns blue. I then add a few drops of sulfuric acid and nothing happens. I add maybe 20 more and the solution loses some of the blue intensity but is still clearly blue. After adding a few tens of mL of the acid, the solution finally turns somewhat clear. There is still no obvious endpoint as when phenolphthalein is used (in other titration experiments I have performed).
According to my stoichiometry, when 10 mL of the stock formaldehyde solution is added to the sodium sulfite solution (this time clear because I added a couple of drops of conc. sulfuric acid), about 25 mL of sulfuric acid (3.) is needed to reach the equivalency point. I added ~160 mL...
I am getting really frustrated with this. Anyone else encounter similar problems?